Interesting find. It brings a few associations to mind.
First, a parallel to the Bible: Jesus says something to the effect that one has to become like a child (and what's more like a child than ignorant and innocent?) to enter the Kingdom of Heaven.
Second, though, is a question or two. The Dunyain are seeking the Absolute, right? So is this a bit of truth that Kellhus is laying on them, i.e. is this something the Dunyain believe or is it something that Kellhus wants the Inrithi to believe so that he can dominate them? Or both, but perhaps in different ways?
Okay, another association: Nietzsche, whose own project is really not so different from the Dunyain, in the sense that he's trying to understand the way that values are imposed on us from outside ourselves and shape us imperceptibly, and then to replace them with values of our own making (theoretically, anyway). He has this concept of three stages of man: the camel, the lion, and the child. The idea, briefly, is that the camel stage is this sort of unawakened pack animal stage: we <i>carry</i> the morality, the values that we've been indoctrinated with. The darkness that comes before us, if you will. The second stage is the lion, where we are aware of the imposition of values and we tear down these morals and values--in other words, recognizing them the way that the Dunyain do as essentially arbitrary, as human-created rather than god-given. And the third stage is the child, where the idea seems to be of a blank slate on which to create a new value system, one that serves the person who takes it on rather than the person serving the value system. That's a convoluted way of suggesting that the Dunyain might actually mean that what Kellhus says about ignorance and innocence, at least in a kind of convoluted sense.
But on another level, like the saying of Jesus referenced above, I suspect that its meaning has more to do with cultivating a child-like trust in Kellhus the authority figure.
Not that any of that helps to justify Serwe as the most important character. I have trouble wrapping my mind around that assertion.